Dry eye syndrome, also known as keratoconjunctivitis sicca, is a multifactorial disorder of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability. Dry eye syndrome is usually caused by inadequate tear production. In such cases, the lacrimal gland does not produce sufficient tears to keep the entire conjunctiva and cornea covered by a complete layer. This typically occurs in people who are otherwise healthy; however, increased age is associated with decreased tearing.
Dry eye syndrome can also be caused by abnormal tear composition resulting in rapid evaporation or premature destruction of the tears. In this condition, although the tear gland produces a sufficient amount of tears, the rate of evaporation of the tears is too rapid. There is a loss of water from the tears that results in tears that are hypertonic. As a result, the entire conjunctiva and cornea cannot be kept covered with a complete layer of tears during certain activities or in certain environments.
Goblet cells are polarized epithelial cells found in columnar and stratified squamous epithelia throughout the body, such as in the conjunctiva. Goblet cells secrete gel-forming mucins that form the mucous layer that protects the wet-surfaced epithelia from the external environment. These cells form the first line of defense between the ocular surface, the inner ear, the gastrointestinal tract and the respiratory tract with the external environment. Goblet cells of the conjunctiva are the primary source of mucus (complex glycoprotein) that constitutes the inner, mucous layer of the tear film. The amount of mucin, as well as its proper hydration and character, is critical to the protection of the epithelia that it overlies. The amount of mucin is controlled by regulating the number of goblet cells, the rate of mucin secretion by the goblet cells, and the rate of mucin synthesis by the goblet cells.